The present invention relates to a metal foil used in a printed wiring board and a process for producing the same, and a process for producing a printed wiring board using said metal foil.
With the progress of electronic instruments, those having higher functions have-been demanded. For example, as to the wiring density, so-called surface mounting parts which are connected to electronic parts only on a surface of wiring board have been developed. A distance of connecting terminal of the electronic part is shortened to 0.15 mm or less. In accordance with such a distance, the formation of circuit conductors for attaining such a density is demanded.
On the other hand, as to heat resistance, the heat resistance of 260.degree. C. which temperature is necessary for soldering is, needless to say, necessary. Further, it is also required to have heat resistance sufficient to withstand under severe conditions of using circumstances, for example, used for controlling a car, or to withstand under bad circumstances of ship bottoms used for transporting products.
Printed wiring boards used for such purposes have been produced by a subtractive process wherein a copper clad laminate obtained by binding a copper foil on an insulating substrate is used as a starting material, and portions of the copper foil not becoming conductor circuits are removed by etching to form circuits, an additive process wherein conductor circuits are formed on an insulating substrate by electroless plating, a partial additive process wherein a part of circuit conductors at inner walls of through holes is formed by electroless plating, etc.
Among these processes, the subtractive process has been carried out for a long time wherein the enhancement of wiring density is usually conducted by thinning the thickness of copper foil used in copper-clad laminate. This is because when unnecessary copper foil protruded from an etching resist is removed by an etching solution after forming the etching resist in the form of necessary circuit on the copper foil surface, a phenomenon of so-called side etching wherein the copper is corroded from side faces of necessary circuit portions takes place. The thicker the copper foil becomes, the larger the removed copper amount from the side faces by the side etching becomes. Thus, the formation of fine circuits becomes difficult.
In addition, in the case of a printed wiring board wherein inner walls of through-holes are metallized to connect different layers of circuits, it is generally conducted to drill holes in a copper-clad laminate, to conduct electroless plating on the whole inner walls of holes and copper foil surfaces, and to conduct electric plating in order to secure the thickness of a metal layer on the inner walls of holes. Thus, the plating layer is formed on the copper foil, resulting in making the copper thickness naturally larger. Therefore, it is necessary to use a thin copper foil when used as the copper-clad laminate which is a starting material. As the thin copper foil, there are used a rolled copper foil obtained by rolling copper with heat and pressure, an electrolyzed copper foil obtained by depositing copper on a surface of metal such as stainless steel by electrolytic plating. The thickness of these foils are usually 18 to 70 .mu.m. Recently, it is also known that an electrolyzed copper foil can be obtained by forming a thin copper foil of about 5 .mu.m thick on an aluminum foil by electrolytic plating.
But such a thin copper foil has problems in that when the copper foil is laminated with non-cured or semi-cured prepreg, handling is so difficult that the copper foil is easily folded by slight force, and such folding often takes place during the production of the copper foil depending on a way of handling. In order to prevent such folding, the thin copper foil is first laminated with a support to form a composite laminate having sufficient strength as a whole, followed by removal of the support after binding with a resin or immediately before the use.
Such a process is disclosed, for example, in U.S. Pat. Nos. 4,394,419 and 4,503,112. According to these U.S. Patents, there is disclosed a metallic foil including (a) a carrier layer of copper fully removable by etching and having a thickness of about 10-35 microns, (b) a thin layer of copper having a thickness in the range of 1-12 microns to be used in the formation of electrical circuit paths, and (c) an intermediate metallic layer positioned between the (a) and (b) layers and secured thereto, and having a thickness in the range of 0.1 to 2.0 microns. But, since the intermediate metallic layer is selected from the group consisting of nickel, a nickel-tin alloy, nickel-iron alloy, lead and a tin-lead alloy, there are many problems in that the intermediate layer is poor in selective etching properties of the carrier copper after heat treatment, some materials are poor in selective etching properties of intermediate layer, and other materials are poor in stability of plating solutions.
Since the selective etching properties of the carrier copper after heat treatment is poor, when the carrier copper is removed after adhesion of laminates, the intermediate layer is also removed sometimes depending on the temperature applied to the laminate adhesion between an insulating layer and such a copper foil.
Further, when a highly heat resistant prepreg such as a polyimide resin-impregnated prepreg or a highly heat resistant thermoplastic such as a fluorine resin is used as intermediate insulating layers in multi-layer wiring boards which are recently required to have high heat resistance, it is necessary to heat at near 400.degree. C. for laminate adhesion. Under such severe conditions, the materials of prior art references lose the selective etching properties of carrier copper to carry out excessive etching, often resulting in damaging the intermediate layer.
As to the selective etching properties of intermediate layer, when a commercially available etching solution comprising ethylenediamine, sodium hydroxide and m-nitrobehzenesulfonic acid as main components is used, an intermediate layer made of nickel, a nickel-iron alloy, nickel-tin alloy or tin is either not etched or causes a so-called "smut" on the surface of intermediate layer. When the smut occurs, the surface of intermediate layer becomes a dull color with irregular light and shade different from a metallic color. Once the smut occurs, it is difficult to remove it and impossible to conduct plating thereon in the production of printed wiring boards. Thus, it is very disadvantageous industrially.
Further, the intermediate layer cannot be removed by only contacting with an etching solution of prior art, and can be removed anodically, i.e. by passing an electric current. But such an electrolytic etching requires a special apparatus, and brings about various problems such as consumption of electric power, maintenance of the apparatus, maintenance of good circumstances, and the like, so that it is not advantageous industrially.
In addition, in the case of a nickel-iron plating solution, there are problems in that the plating solution per se is not stable, nickel-iron plating is not advantageous industrially, and the like. The tin plated layer obtained by using an acidic tin plating solution can be formed as an intermediate layer, but when it is subjected to copper plating for forming circuit copper, it contaminates the copper plating solution. This is not advantageous industrially.
On the other hand, printed wiring boards are recently required to have both high wiring density and high heat resistance. In order to use a resin having high heat resistance, it is necessary to use materials which are not denatured at higher curing temperatures in the production of printed wiring boards since the curing temperature becomes very high. For example, when a high heat resistant polyimide resin is used together with the above-mentioned three-layered metal foil, the three-layered metal foil is also required to have high heat resistance during the production procedure due to a high temperature used for curing the high heat resistant resin. This heat resistance not only means that no physical changes such as deformation, shrinkage and expansion due to high temperatures take place but also requires that the intermediate layer among the three layers should not be denatured at high temperatures, and the copper layer used as the support should be removed easily. But according to known techniques heretofore developed, denaturing of the intermediate layer at high temperatures cannot be avoided and it is difficult to easily remove the composite layer.